1. Field of the Invention
The present invention relates to a centrifugal fan, and particularly a centrifugal fan in which noises attributable to air blowing are reduced and also cost is reduced.
2. Description of the Related Art
A centrifugal fan is structured to include: a scroll casing which includes an air inlet opening and an air outlet opening; and an impeller which is housed in the scroll casing. The impeller is structured such that a number of blades are arranged around the circumference of a rotary shaft of a motor.
The centrifugal fan operates in such a manner that air is taken in through the air inlet opening of the scroll casing, moved from the center of the impeller through between the blades and then emitted in the radially outward direction of the impeller by hydrodynamic force generated by centrifugal action resulting from the rotation of the impeller. The air emitted out from the outer circumference of the impeller is caused to travel inside the scroll casing and is blown out through the air outlet opening as high-pressure air.
The centrifugal fan is widely used for cooling household electric appliances, office automation equipment or industrial equipment, for air ventilation or air conditioning, and is also used as an air blower for vehicles. The blast performance and noise level of the centrifugal fan are largely influenced by the blade shape of the impeller and the shape of the scroll casing (namely, the structure of the centrifugal fan).
Various approaches have conventionally been made for optimizing the impeller blade shape and the scroll casing structure for the purpose of reducing the noise level and enhancing the blast performance. In one conventional approach, a centrifugal fan is proposed in which the shape of impeller blades is optimized to thereby reduce noise level (refer to, for example, Japanese Patent Application Laid-Open No. S63-289295).
FIG. 6 shows in plan a cutaway view of the aforementioned centrifugal fan disclosed in Japanese Patent Application Laid-Open No. S63-289295, and FIG. 7 shows perspectively an impeller of the centrifugal fan shown in FIG. 6.
Referring to FIGS. 6 and 7, an impeller 120 of a centrifugal fan 100 includes a primary plate 121, a secondary plate 122, and a plurality of blades 123 arranged between the primary plate 121 and the secondary plate 122. When the impeller 120 rotates (in the counterclockwise direction in FIG. 6), the outer circumferential portion of the blade 123 rotates behind the inner circumferential portion of the blade 123. The impeller 120 is attached inside a casing 127 having a scroll shape, and air is blown.
The air to be blown is taken in through an air inlet opening 140 and emitted from the outer circumference of the impeller 120 by hydrodynamic force generated by centrifugal action by means of the blades 123 of the impeller 120. The pressure of the air emitted out from the outer circumference of the impeller 120 is increased by means of the scroll-shaped casing 127 and then is blown out through an air outlet opening 141.
The impeller 120 thus has what is called a backward inclined blade structure in which, as described above, the outer circumferential portion of the blade 123 rotates behind the inner circumferential portion of the blade 123 with respect to the rotation direction of the impeller 120. Namely, as shown in FIG. 6, the blade 123 is curved to be inclined backward with respect to the rotation direction of the impeller 120. A centrifugal fan which is provided with the impeller blade structure described above is generally called a turbo-fan.
In the turbo-fan (centrifugal fan 100) shown in FIG. 6, the plurality of blades 123 are sandwiched between the primary plate 121 and the secondary plate 122 which has a diameter identical to the diameter of the primary plate 121.
The blade 123 has a trailing edge obliquely cut off so that a curved side 135 positioned at the primary plate 121 is shorter than a curved side 134 positioned at the secondary plate 122. As a result, a time lag is generated between a time when a trailing edge portion 131 positioned at the primary plate 121 passes by a casing tongue 129 and a time when a trailing edge portion 132 positioned at the secondary plate 122 passes by the casing tongue 129.
Consequently, as described in Japanese Patent Application Laid-Open No. S63-289295, the pressure variation caused by the blade 123 passing by the casing tongue 129 can be temporally dispersed and so the energy to produce sound is dispersed thus reducing generation of noises.
In the above turbo-fan described in Japanese Patent Application Laid-Open No. S63-289295, the shape of the blade 123 is optimally modified in order to reduce noises produced when air is blown. In the scroll-shaped casing 126, however, the casing tongue 129 is formed, and noises which are produced when the air emitted from the impeller hits the casing tongue 129 cannot be satisfactorily reduced.
Also, in the turbo-fan described in Japanese Patent Application Laid-Open No. S63-289295, the air emitted from the outer circumference of the impeller 120 is caused to travel along the inner surface of the scroll-shaped casing 127 and to be blown out through the air outlet opening 141. Therefore, the air flow is likely to be disturbed at the inner surface of the scroll-shaped casing 127 and also near the air outlet opening, and the air flow disturbed there causes noise generation.
Further, recently, for the centrifugal fan used in an air blower for household electric appliances, office automation equipment or vehicles, cost reduction as well as noise reduction and miniaturization is strongly requested.
In the scroll-shaped casing 127 shown in FIG. 6, however, an air flow passage to conduct air to the air outlet opening 141 must be provided around the outer circumference of the impeller 120. Accordingly, the outer diameter of the scroll-shaped casing 127 has to be about twice as large as the outer diameter of the impeller 120, which, as a result, makes it difficult to downsize the turbo-fan.
The roll-scrolled casing 127 and the impeller 120 are generally made of synthetic resin, and due to the recent economic situation that the price of synthetic resin is increasing, there is a problem, in addition to the above described downsizing difficulty problem, that the cost of the centrifugal fan cannot be easily reduced.